Frequency converter and communication device

ABSTRACT

The frequency conversion circuit  30  as an element of a radio communication device, the frequency conversion circuit for converting by using a local oscillator  32  the frequency of a signal which is modulated with a reference oscillation signal from a reference oscillator  31.  The frequency conversion circuit  30  is provided with a first mixer  36,  a change-over switch  35,  and a second mixer  33.  The first mixer  36  mixes the reference oscillation signal from the reference oscillator  31  and a local oscillation signal from the local oscillator. The change-over switch  35  selects one of a mixed signal from the first mixer  36  and the local oscillation signal from the local oscillator  32.  The second mixer  33  mixes the signal modulated with the reference oscillation signal with an output from the change-over switch  35.  The frequency conversion circuit  30,  thus configured, may widen a frequency range, without an additional local oscillator, guaranteeing signal purity and response time for shifting the oscillation frequency.

TECHNICAL FIELD

[0001] This invention relates to a frequency conversion circuit used insuch as a radio communication system. The frequency conversion circuitcarries out frequency conversion by using a reference oscillator and alocal oscillator. This invention also relates to a communication deviceemploying this frequency conversion circuit.

BACKGROUND ART

[0002]FIG. 10 is a block diagram illustrating the electricalconfiguration of a radio communication device as a first conventionalexample. This radio communication device is provided with a referenceoscillator 1 for outputting the reference oscillation signal at aspecific frequency, a local oscillator 2 whose oscillation frequency isvariable, a mixer 3 for transmission, a quadrature modulator 5 forproducing quadrature modulation on the I channel and the Q channel of abaseband signal, an antenna 6 for radio transmission and reception, amixer 7 for reception, a signal processing circuit 8 for performingvarious types of signal processing on baseband (BB) transmission andreception signals, and so forth. Then, the local oscillator 2, the mixer3 and the mixer 7 form a frequency conversion circuit.

[0003] In the case of this radio communication device transmitting asignal, initially, a baseband signal (the I channel and the Q channel)for transmission is generated in the signal processing circuit 8. Then,the generated baseband signal is subjected to quadrature modulation bymeans of a reference oscillation signal which is outputted from thereference oscillator 1. The signal subjected to quadrature modulation isinputted to the mixer 3 by way of a bandpass filter (BPF) and anamplifier (AMP), and then up-converted by means of a local oscillationsignal which is outputted from the local oscillator 2 to become a highfrequency signal to be adaptable for radio transmission. Theup-converted signal then passes through another bandpass filter, anotheramplifier, a low-pass filter (LPF) and a duplexer to reach the antenna 6to be outputted through radio transmission.

[0004] Then, upon reception of a signal, a signal arrived at the antenna6 passes through the duplexer, a bandpass filter, a low noise amplifier(LNA) and another bandpass filter then to be inputted to the mixer 7.The mixer 7, also receiving a local oscillation signal from the localoscillator 2, down-converts the received signal into a baseband signal(the I channel, the Q channel). The down-converted signal passes througha low-pass filter and a baseband amplifier (BB-AMP) then to be inputtedto the signal processing circuit 8.

[0005] Thus, the radio communication device of FIG. 10 generates a radiotransmission signal by using the reference oscillator 1 and thefrequency modulation circuit (including the local oscillator 2), andalso generates a baseband reception signal from a radio signal by usingthe frequency modulation circuit including the local oscillator 2.

[0006] According to a first conventional example, a frequency range maybe widened only by widening the frequency variable range of the localoscillator. However, because there is a trade-off between the frequencyvariable range of the local oscillator and the C/N deterioration and thelike of an output signal, widening the frequency variable range resultsin deteriorating the signal purity. In addition to that, in the case ofwidening the frequency variable range in a single local oscillator, theresponse time for shifting an oscillation frequency is caused to becomelong. In particular, in the case of shifting from a minimum frequency toa maximum frequency, or in the case of shifting from a maximum frequencyto a minimum frequency, the response time becomes long. For that reason,in order to guarantee an acceptable degree of signal purity and theresponse time for shifting the oscillation frequency, the frequencyvariable range has to be limited.

[0007]FIG. 11 is a block diagram illustrating a frequency conversioncircuit as an element of a second conventional example. A radiocommunication device according to the second conventional example isconfigured almost in the same manner as that of FIG. 10 with an onlydifference in the configuration of the frequency conversion circuit.Specifically, the frequency conversion circuit is provided with localoscillators 2 a, 2 b whose oscillation frequencies are variable, themixer 3 for transmission and a change-over switch 4. The change-overswitch 4 selects one of outputs from the local oscillators 2 a, 2 b andthen outputs it to the mixer 3. According to the second conventionalexample, a radio transmission signal is generated by using the referenceoscillator 1 and the frequency conversion circuit (including the localoscillators 2 a, 2 b).

[0008] The second conventional example thus uses two local oscillators,thereby guaranteeing the response time for shifting the oscillationfrequency for the respective local oscillators. However, the need ofhaving the additional local oscillator has posed a problem of increasingthe number of components as well as the scale of the circuit.

DISCLOSURE OF THE INVENTION

[0009] An object of this invention is to provide a frequency conversioncircuit and a communication device which may widen the frequencyvariable range, without an additional local oscillator, whenguaranteeing the signal purity and the response time for shifting theoscillation frequency.

[0010] This invention is directed to a frequency conversion circuitwhich converts the frequency of a signal, which is modulated with areference oscillation signal from a reference oscillator, by means of alocal oscillator.

[0011] The frequency conversion circuit is characterized with a firstmixer for mixing the reference oscillation signal from the referenceoscillator and a local oscillation signal from the local oscillator; aswitch for selecting one of a mixed signal from the first mixer and thelocal oscillation signal from the local oscillator; and a second mixerfor mixing the signal which is modulated with the reference oscillationsignal with a signal selected by the switch.

[0012] According to this invention, the reference oscillator which isused for signal modulation is diverted for frequency conversion. Forthat reason, another additional local oscillator other than the localoscillator is not required. Furthermore, the frequency conversion isperformed by selecting one of a mixed signal from the referenceoscillator and the local oscillator and a single signal of the localoscillation signal from the local oscillator by the switch. For thatreason, the band may be widened without widening the frequency variablerange of the local oscillator nor deteriorating the signal purity. Stillmore, frequencies are changed by the switch. This may prevent theresponse time for shifting frequencies from increasing.

[0013] Further, the present invention is characterized with a bandpassfilter, which is provided after the first mixer, for eliminatingspurious which occurs in the first mixer.

[0014] According to this invention, it is allowed to eliminate spuriouswhich may occur when mixing a reference oscillation signal and a localoscillation signal in the first mixer. For that reason, the signalpurity may be prevented from being degraded.

[0015] Still further, the present invention is characterized with afrequency divider, which is placed between the reference oscillator andthe first mixer, for dividing a frequency of the reference oscillationsignal.

[0016] According to this invention, it is allowed by having thefrequency divider for dividing the frequency of a reference oscillationsignal that the frequency conversion circuit is capable of handlingsignals in other frequency variable ranges.

[0017] Still further, the present invention is characterized that thefrequency divider is a variable frequency divider whose frequencydivision ratio is variable.

[0018] According to this invention, it is allowed by varying thefrequency division ratio of the frequency divider to vary the frequencyvariable range.

[0019] Still further, the present invention is characterized furtherwith a bandpass filter, which is provided after the frequency divider,for eliminating spurious which occurs in the frequency divider.

[0020] According to this invention, it is allowed to eliminate spuriouswhich may occur when dividing the frequency of a reference oscillationsignal in the frequency divider. For that reason, the signal purity maybe prevented from being degraded.

[0021] Still further, the present invention is characterized furtherwith a multiplier, which is provided between the reference oscillatorand the first mixer, for multiplying a frequency of the referenceoscillation signal.

[0022] According to this invention, it is allowed by having themultiplier for multiplying the frequency of the reference oscillationsignal that the frequency conversion circuit is capable of handlingsignals in other frequency variable ranges.

[0023] Still further, the present invention is characterized furtherwith a bandpass filter, which is provided after the multiplier, foreliminating spurious which occurs in the multiplier.

[0024] According to this invention, it is allowed to eliminate spuriouswhich may occur when multiplying the frequency of a referenceoscillation signal in the multiplier. For that reason, the signal puritymay be prevented from being degraded.

[0025] Furthermore, the present invention is directed to a communicationdevice, which includes a frequency conversion circuit for up-convertinga signal which is modulated with a reference oscillation signal from areference oscillator by using a local oscillator, the communicationdevice for transmitting a frequency converted signal by the frequencyconversion circuit to outside.

[0026] The communication device is characterized with the frequencyconversion circuit which includes a first mixer for mixing the referenceoscillation signal from the reference oscillator and a local oscillationsignal from the local oscillator; a switch for selecting one of a mixedsignal from the first mixer and the local oscillation signal from thelocal oscillator; and a second mixer for mixing the signal which ismodulated with the reference oscillation signal with a signal selectedby the switch.

[0027] According to this invention, another additional local oscillatoris not required similar to that of the frequency conversion circuitabove. Furthermore, the variable frequency range may be widened,guaranteeing the signal purity and the response time for shiftingoscillation frequencies.

[0028] Still further, the present invention is characterized that thefrequency conversion circuit further includes a third mixer for mixing asignal received from outside with an output from the switch and fordown-converting.

[0029] According to this invention, the frequency conversion circuitwhich is used for converting the frequency of a transmission signal isshared on the reception side, which allows the reception side toeliminate the necessity of having a special reference oscillator and aspecial local oscillator for the reception side, which simplifies theconfiguration of the communication device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a block diagram illustrating the electricalconfiguration of a radio communication device as a first embodiment.

[0031]FIG. 2 is a block diagram of a frequency conversion circuit as anelement of a second embodiment.

[0032]FIG. 3 is a block diagram of a frequency conversion circuit as anelement of a third embodiment.

[0033]FIG. 4 is a block diagram of a frequency conversion circuit as anelement of a fourth embodiment.

[0034]FIG. 5 is a block diagram of a frequency conversion circuit as anelement of a fifth embodiment.

[0035]FIG. 6 is a block diagram of a frequency conversion circuit as anelement of a sixth embodiment.

[0036]FIG. 7 is a block diagram of a frequency conversion circuit as anelement of a seventh embodiment.

[0037]FIG. 8 is a block diagram of a frequency conversion circuit as anelement of an eighth embodiment.

[0038]FIG. 9 is a block diagram of a frequency conversion circuit as anelement of a ninth embodiment.

[0039]FIG. 10 is a block diagram illustrating the electricalconfiguration of a radio communication device as a first conventionalexample.

[0040]FIG. 11 is a block diagram of a frequency conversion circuit as anelement of a second conventional example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0041] The embodiments of the present invention will be discussed belowwith reference to the drawings.

[0042] (First Embodiment)

[0043]FIG. 1 is a block diagram illustrating an electrical configurationof a radio commination device as a first embodiment. This radiocommunication device is provided with a quadrature modulator 10,bandpass filters (BPF) 11, 13, 18, 20, amplifiers 12, 14, low-passfilters (LPF) 15, 22, an antenna 16, a duplexer 17, a low noiseamplifier (LNA) 19, a baseband amplifier (BB-AMP) 23, a signalprocessing circuit 24, and a frequency conversion circuit 30. Thequadrature modulator 10 is a modulator which mixes a baseband signal(the I channel, the Q channel) sent from the signal processing circuit24 with the reference oscillation signal of a reference oscillator 31and carried out the quadrature modulation. The bandpass filter 11eliminates an unnecessary signal which may occur through the quadraturemodulation carried out by the quadrature modulator 10. The amplifier 12amplifies a signal passed through the bandpass filter 11. The bandpassfilter 13 eliminates an unnecessary signal which may occur when a signalamplified in the amplifier 12 and a selected local oscillation signal bya change-over switch 35 (explained later) are mixed in the mixer 33 tobe converted into a high frequency signal. The amplifier 14 amplifies asignal passed through the bandpass filter 13. The low-pass filter 15eliminates a harmonic distortion which may occur through theamplification of a signal in the amplifier 14.

[0044] The duplexer 17 is a filter provided for allowing the singleantenna 16 to be shared for transmission and reception, and used forpreventing a signal passing through one channel from entering the otherchannel for transmission or reception. The duplexer 17 is formed bycombining the transmission filter (BPF) which lets the signals of thetransmission band only pass through and the reception filter (BPF) whichlets the signals of the reception band only pass through.

[0045] The bandpass filter 18 eliminates signals other than those of thereception band from signals received through the antenna 16. The lownoise amplifier 19 amplifies a signal passed through the bandpass filter18. The bandpass filter 20 eliminates an unnecessary signal (including aharmonic distortion) which may occur through the amplification of thesignal in the low noise amplifier 19. The low pass filter 22 eliminatesan unnecessary harmonic distortion which may occur in the mixer 34. Thebase band amplifier 23 amplifies a baseband signal passed through thelow-pass filter 22.

[0046] The signal processing circuit 24 is a circuit provided forcarrying out various types of signal processing on transmitting andreceived baseband signals.

[0047] When this radio communication device transmits a signal, firstly,a baseband signal (the I channel, the Q channel) for transmission isgenerated in the signal processing circuit 24. Then, the generatedbaseband signal is subjected to quadrature modulation by using areference oscillation signal which is outputted from the referenceoscillator 31. The signal subjected to quadrature modulation is theninputted to the mixer 33 through the bandpass filter 11 and theamplifier 12 according to this order. A signal from the amplifier 12 ismixed in the mixer 33 with a local oscillation signal from the localoscillator 32, and then up-converted into a high frequency signal to beadaptable for radio transmission. Then, the up-converted signal passesthrough the bandpass filter 13, the amplifier 14, the low-pass filter 15and the duplexer 17 according to this order then to be transmittedthrough the antenna 16.

[0048] Upon reception of a signal, on the other hand, a signal receivedthrough the antenna 16 passes through the duplexer 17, the bandpassfilter 18, the low noise amplifier 19, and the bandpass filter 20according to this order then to be inputted to the mixer 34. The signalfrom the bandpass filter 20 is mixed in the mixer 34 with a localoscillation signal from the local oscillator 32 to be down-convertedinto a baseband signal (the I channel, the Q channel). Thedown-converted signal passes through the low-pass filter 22 and thebaseband amplifier 23 according to this order, then to be sent to thesignal processing circuit 24.

[0049] The frequency conversion circuit 30 is explained below. Thefrequency conversion circuit 30 is a circuit provided for up-convertinga signal modulated in the quadrature modulator 10 into a radiotransmission frequency signal and down-converting a received radiosignal into a baseband signal. The frequency conversion circuit 30 isprovided with the reference oscillator 31, the local oscillator 32, themixers 33, 34, 36, the change-over switch 35, and a frequency divider37.

[0050] The reference oscillator 31 is a local oscillator using thefrequency of an output signal at a given fixed frequency. For example,the reference oscillator 31 outputs a signal at the frequency off_(IF)=360 MHz. The local oscillator 32 is a local oscillator whoseoscillation frequency can be variable, which outputs a signal atf_(Lo1)=1750˜1810 MHz, for example. Those local oscillators are PLL(Phase Locked Loop) circuits. The mixer 33 (a second mixer) mixes asignal from the amplifier 12 with a signal from the change-over switch35. The mixer 34 (a third mixer) mixes a reception signal passed throughthe bandpass filer 20 with a signal selected by the change-over switch35. The change-over switch 35 selects one of signals from the localoscillator 32 and the mixer 36, and then inputs a selected signal to themixer 33 and the mixer 34. The mixer 36 (a first mixer) mixes a signaloutputted from the reference oscillator 31 via the frequency divider 37with a signal from the local oscillator 32. The frequency divider 37,which is placed between the reference oscillator 31 and the mixer 36,divides the oscillation frequency of the reference oscillator 31according to a fixed frequency division ratio ⅙, and then outputs thesignal at a divided frequency.

[0051] According to this frequency conversion circuit 30, the frequency,f_(Lo1)=1750˜1810 MHz, of an oscillation signal which is outputteddirectly from the local oscillator 32 is a first local oscillationsignal frequency. Then, the reference frequency f_(IF)=360 MHz by thereference oscillator 31 is divided by the frequency divider 37 to become⅙ of the reference frequency, i.e., 60 MHz, which is now mixed with theoscillation frequency, f_(LO1)=1750˜1810 MHz, of the local oscillator 32to become a mixed frequency, 1810˜1870 MHz. This mixed frequency of1810˜1870 MHz is the second local oscillator signal frequency. Of thefirst local oscillation signal frequency and the second localoscillation signal frequency, either one is selected by the change-overswitch 35. As a result, even if the frequency variable range of thelocal oscillator 32 is 1750˜1810 MHz, the frequency conversion circuit30, as a whole, is still capable of handling signals in the wider rangeof frequencies of 1750˜1870 MHz.

[0052] Thus, according to the first embodiment, the reference oscillator31 which is used for signal modulation is now diverted for frequencyconversion. For that reason, another additional local oscillator otherthan the local oscillator 32 is not required. Furthermore, the frequencyconversion is performed by selecting one of a mixed signal from thereference oscillator 31 and the local oscillator 32 and a single signalof the local oscillation signal from the local oscillator 32 by thechange-over switch 35. For that reason, the band may be widened withoutwidening the frequency variable range of the local oscillator 32 nordeteriorating the signal purity. Still more, frequencies are changed bythe change-over switch 35. This may serve for quick frequency shifting.On top of it, if the frequency setting of the local oscillator 32 andthe switching operation of the change-over switch 35 are carried out atthe same time, quicker frequency shifting can be obtained.

[0053] It is to be noted that although this embodiment employs the LPF15 and the BPF 18, if a sufficient performance is achieved by theduplexer 17, and in other words, if the duplexer 17 sufficientlyeliminates signals other than those in the frequency range fortransmission from transmission signals as well as signals other thanthose in the frequency range for reception from reception signals, thenthe LPF 15 and the BPF 18 may be omitted.

[0054] Upon discussion of a second embodiment through a ninth embodimentgiven below, it is to be noted that each of those embodiments employsthe same radio communication device as that of the first embodimentexcept for the frequency conversion circuit. For this reason, only thefrequency conversion circuits will be explained in those embodiments,and the other parts of which will not be explained. Furthermore, themixer 34 for mixing a signal from the change-over switch 35 with areception signal is provided in the respective embodiments of the secondembodiment through the ninth embodiment, but the mixer 34 is not shownin any of FIG. 2 through FIG. 9.

[0055] (Second Embodiment)

[0056]FIG. 2 is a block diagram of a frequency conversion circuit as anelement of a second embodiment. A frequency conversion circuit 42modifies the frequency conversion circuit 30 (FIG. 1) by replacing thefrequency divider 37 by a multiplier 52. The multiplier 52 is placedbetween the reference oscillator 31 and the mixer 36, and multiplies theoscillation frequency of the reference oscillator 31 by a fixedmultiplication ratio M. As a result, the frequency conversion circuit 42is adaptable to handling frequencies in a variable range different fromthe frequency variable range of 1750˜1870 MHz which is applied to thefrequency conversion circuit 30 (FIG. 1).

[0057] The configuration of the frequency conversion circuit 42 is thesame as that of the frequency conversion circuit 30 (FIG. 1) except forthe multiplier 52. For that reason, the second embodiment also allowsthe variable frequency range to be widened, without an additional localoscillator, guaranteeing the signal purity and the response time forshifting the oscillation frequency, as discussed in the firstembodiment.

[0058] (Third Embodiment)

[0059]FIG. 3 is a block diagram of a frequency conversion circuit as anelement in a third embodiment. A frequency conversion circuit 43modifies the frequency conversion circuit 30 (FIG. 1) by adding abandpass filter 53 between the frequency divider 37 and the mixer 36.The bandpass filter 53 is a filter provided for eliminating spuriouswhich may occur when the frequency divider 37 divides the frequency ofthe reference oscillation signal. Thus, the spurious occurred in thefrequency divider 37 is eliminated immediately after it occurred, andtherefore unnecessary spurious is not allowed to remain behind after themixer 36. As a result, the signal purity may be kept in a goodcondition.

[0060] The configuration of the frequency conversion circuit 43 is thesame as that of the frequency conversion circuit 30 (FIG. 1) except forthe additionally provided bandpass filter 53. For that reason, the thirdembodiment also allows the variable frequency range to be widened,without an additional local oscillator, guaranteeing the signal purityand the response time for shifting the oscillation frequency, asdiscussed in the first embodiment.

[0061] It is to be noted that the bandpass filter 53 is employed in thisembodiment, but a low-pass filter may be used instead in the case ofonly few occurrences of low frequency signals which are lower than thoseobtained after frequency division by the frequency divider 37.

[0062] (Fourth Embodiment)

[0063]FIG. 4 is a block diagram of a frequency conversion circuit as anelement of a fourth embodiment. A frequency conversion circuit 44modifies the frequency conversion circuit 30 (FIG. 1) by adding abandpass filter 54 between the mixer 36 and the change-over switch 35.The bandpass filter 54 is a filter provided for eliminating spuriouswhich may occur when the mixer 36 mixes a signal from the frequencydivider 37 and a signal from the local oscillator 32. Thus, the spuriousoccurred in the mixer 36 is eliminated immediately after it occurred,and therefore unnecessary spurious is not allowed to remain behind afterthe change-over switch 35. As a result, the signal purity may bemaintained in a good condition.

[0064] The configuration of the frequency conversion circuit 44 is thesame as that of the frequency conversion circuit 30 (FIG. 1) except forthe additionally provided bandpass filter 54. For that reason, thefourth embodiment also allows the variable frequency range to bewidened, without an additional local oscillator, guaranteeing the signalpurity and the response time for shifting the oscillation frequency, asdiscussed in the first embodiment.

[0065] (Fifth Embodiment)

[0066]FIG. 5 is a block diagram of a frequency conversion circuit as anelement of a fifth embodiment. A frequency conversion circuit 45modifies the frequency conversion circuit 30 (FIG. 1) by adding both ofthe bandpass filter 53 (FIG. 3) and the bandpass filter 54 (FIG. 4). Inother words, the bandpass filter 53 being provided between the frequencydivider 37 and the mixer 36 serves for eliminating spurious which mayoccur in the frequency divider 37. Also, the bandpass filter 54 beingprovided between the mixer 36 and the change-over switch 35 serves foreliminating spurious which may occur in the mixer 36. Thus, the spuriousoccurred in the frequency divider 37 is eliminated immediately after itoccurred, and therefore unnecessary spurious is not allowed to remainbehind after the mixer 36. Similarly, the spurious occurred in the mixer36 is eliminated immediately after it occurred, and thereforeunnecessary spurious is not allowed to remain behind after thechange-over switch 35. As a result, the signal purity may be kept in agood condition

[0067] The configuration of the frequency conversion circuit 45 is thesame as that of the frequency conversion circuit 30 (FIG. 1) except forthe additionally provided bandpass filters 53 and 54. For that reason,the fifth embodiment also allows the variable frequency range to bewidened, without an additional local oscillator, guaranteeing the signalpurity and the response time for shifting the oscillation frequency, asdiscussed in the first embodiment.

[0068] (Sixth Embodiment)

[0069]FIG. 6 is a block diagram of a frequency conversion circuit as anelement of a sixth embodiment. A frequency conversion circuit 46modifies the frequency conversion circuit 30 (FIG. 1) by replacing thefrequency divider 37 by a variable frequency divider 56. The frequencydivision ratio of the variable frequency divider 56 is variable whereasthe frequency division ratio of the frequency divider 37 is fixed (⅙).By varying the frequency division ratio of this variable frequencydivider 56, the variable range of frequency is allowed to be furtherwidened.

[0070] The configuration of the frequency conversion circuit 46 is thesame as that of the frequency conversion circuit 30 (FIG. 1) except forthe variable frequency divider 56. For that reason, the sixth embodimentalso allows the variable frequency range to be widened, without anadditional new local oscillator, guaranteeing the signal purity and theresponse time for shifting the oscillation frequency, as discussed inthe first embodiment.

[0071] (Seventh Embodiment)

[0072]FIG. 7 is a block diagram of a frequency conversion circuit as anelement of a seventh embodiment. A frequency conversion circuit 47modifies the frequency conversion circuit 42 (FIG. 2) by adding abandpass filter 57 between the multiplier 52 and the mixer 36. Thebandpass filter 57 is a filter provided for eliminating spurious whichmay occur when the multiplier 52 multiplies the reference oscillationsignal. Thus, the spurious occurred in the multiplier 52 is eliminatedimmediately after it occurred, and therefore unnecessary spurious is notallowed to remain behind after the mixer 36. As a result, the signalpurity may be maintained in a good condition.

[0073] The configuration of the frequency conversion circuit 44 is thesame as that of the frequency conversion circuit 42 (FIG. 2) except forthe additionally provided bandpass filter 57. For that reason, theseventh embodiment also allows to widen the range of variable frequency,without requiring an additional new local oscillator, guaranteeing thesignal purity and the response time for shifting the oscillationfrequency, as discussed in the second embodiment.

[0074] It is to be noted that the bandpass filter 57 is employed in thisembodiment, but a low-pass filter may be used instead in the case ofonly few occurrences of low frequency signals which are lower than thoseobtained after multiplication by the multiplier 52.

[0075] (Eighth Embodiment)

[0076]FIG. 8 is a block diagram of a frequency conversion circuit as anelement of an eighth embodiment. The frequency conversion circuit 48modifies the frequency conversion circuit 42 (FIG. 2) by adding abandpass filter 58 between the mixer 36 and the change-over switch 35.The bandpass filter 58 is a filter provided for eliminating spuriouswhich may occur when the mixer 36 mixes a signal from the multiplier 52and a local oscillation signal from the local oscillator 32. Thus, thespurious occurred in the mixer 36 is eliminated immediately after itoccurred, and therefore unnecessary spurious is not allowed to remainbehind after the change-over switch 35. As a result, the signal puritymay be maintained in a good condition.

[0077] The configuration of the frequency conversion circuit 48 is thesame as that of the frequency conversion circuit 42 (FIG. 2) except forthe additionally provided bandpass filter 58. For that reason, theeighth embodiment also allows the variable frequency range to bewidened, without an additional new local oscillator, guaranteeing thesignal purity and the response time for shifting the oscillationfrequency, as discussed in the second embodiment.

[0078] (Ninth Embodiment)

[0079]FIG. 9 is a block diagram of a frequency conversion circuit as anelement of a ninth embodiment. A frequency conversion circuit 49modifies the frequency conversion circuit 42 (FIG. 2) by adding both ofthe bandpass filter 57 (FIG. 7) and the bandpass filter 58 (FIG. 8). Inother words, the bandpass filter 57 being provided between themultiplier 52 and the mixer 36 serves for eliminating spurious which mayoccur in the multiplier 52. Similarly, the bandpass filter 58 beingprovided between the mixer 36 and the change-over switch 35 serves foreliminating spurious which may occur in the mixer 36. Thus, the spuriousoccurred in the multiplier 52 is eliminated immediately after itoccurred, and therefore unnecessary spurious is not allowed to remainbehind after the mixer 36. Similarly, the spurious occurred in the mixer36 is thus eliminated immediately after it occurred, and thereforeunnecessary spurious is not allowed to remain behind after thechange-over switch 35. As a result, the signal purity may be kept in agood condition

[0080] The configuration of the frequency conversion circuit 49 is thesame as that of the frequency conversion circuit 42 (FIG. 2) except forthe additionally provided bandpass filters 57 and 58. For that reason,the ninth embodiment also allows the variable frequency range, withoutan additional local oscillator, guaranteeing the signal purity and theresponse time for shifting the oscillation frequency, as discussed inthe second embodiment.

[0081] Industrial Applicability

[0082] The frequency conversion circuit and the communication deviceaccording to the present invention discussed above are adaptable to aradio communication system, a base transceiver station, and a radioterminal device of such as the CDMA system.

1. A frequency conversion circuit for converting a frequency of a signalby using a local oscillator, the signal being modulated with a referenceoscillation signal which is generated by a reference oscillator, thefrequency conversion circuit comprising: a first mixer for mixing thereference oscillation signal generated by the reference oscillator and alocal oscillation signal generated by the local oscillator; a switch forselecting one of a mixed signal received from the first mixer and thelocal oscillation signal received from the local oscillator; and asecond mixer for mixing the signal which is modulated with the referenceoscillation signal with a signal selected by the switch.
 2. Thefrequency conversion circuit according to claim 1, comprising a bandpassfilter, which is placed after the first mixer, for eliminating spuriouswhich occurs in the first mixer.
 3. The frequency conversion circuitaccording to claim 1 or claim 2, further comprising a frequency divider,which is placed between the reference oscillator and the first mixer,for dividing a frequency of the reference oscillation signal.
 4. Thefrequency conversion circuit according to claim 3, wherein the frequencydivider is a variable frequency divider whose frequency division ratiois variable.
 5. The frequency conversion circuit according to claim 3,comprising a bandpass filter, which is placed after the frequencydivider, for eliminating spurious which occurs in the frequency divider.6. The frequency conversion circuit according to claim 1 or claim 2,comprising a multiplier, which is placed between the referenceoscillator and the first mixer, for multiplying a frequency of thereference oscillation signal.
 7. The frequency conversion circuitaccording to claim 6, further comprising a bandpass filter, which isplaced after the multiplier, for eliminating spurious which occurs inthe multiplier.
 8. A communication device, which includes a frequencyconversion circuit for up-converting a signal which is modulated with areference oscillation signal received from a reference oscillator byusing a local oscillator, the communication device for transmitting afrequency converted signal by the frequency conversion circuit tooutside, wherein the frequency conversion circuit includes, a firstmixer for mixing the reference oscillation signal from the referenceoscillator and a local oscillation signal from the local oscillator, aswitch for selecting one of a mixed signal received from the first mixerand the local oscillation signal received from the local oscillator, anda second mixer for mixing the signal which is modulated with thereference oscillation signal with a signal selected by the switch. 9.The communication device according to claim 8, wherein the frequencyconversion circuit further includes a third mixer for mixing a signalreceived from outside with an output from the switch and fordown-converting.